This invention relates to a thermo-sensitive reed switch having permanent magnets as the source of magnetic flux to be varied with temperature, the magnetic flux therefrom being controlled in such a way as to change the intensity and distribution of the magnetic field acting on the switch element by a thermo-sensitive magnetic element whose saturation flux density falls with rising temperature and, in the neighborhood of its Curie temperature, drops sharply to turn the magnetic element into a paramagnetic state, thereby actuating the contact points of the switch element to close and open in on-off manner.
A thermo-sensitive reed switch uses, as its switch element, a glass tube hermetically containing the contact part. Because of this construction, the switch is more durable than bi-metal switches in terms of contact point wear and switch aging, and is therefore more reliable. Notwithstanding this advantage, its temperature hysteresis, that is, the difference between operating or opening temperature and resetting or closing temperature is small as compared with those of temperature switches of bi-metal and liquid expansion types. Small temperature hysteresis implies higher accuracy of response to controlling temperature, but is likely to result in a switch repeating the cycle of closing and opening actions with a higher frequency, leading to wear of contact points, changing of contact resistance and, eventually, giving rise to such operating difficulties as contact point fusion or imperfect contact point separation. Because of these drawbacks and advantages of thermo-sensitive reed switches in general, switches having such large temperature hysteresis as will minimize the on-off frequency within the allowable range of control temperature variation have come to be preferred. Such reed switches last long in service.
Reed switches such as those proposed in Japanese utility model application No. SHO 51-26279, published July 3, 1976 (SHOWA 51), have been developed to meet this need for long-life thermo-sensitive reed switches, but production of such switches has been found to involve a problem in that, the larger the temperature hysteresis, the greater the variation among produced switches of the temperature levels at which opening and closing actions take place.
The present invention is directed to an improvement of reed switches of the type shown in the Japanese utility model application cited above. In particular, the thermo-sensitive magnetic element is slit, preferably parallel to the reeds in the element, thereby greatly improving performance.
The object of this invention is to provide a reed switch with large temperature hysteresis, whose operating temperature can be brought closer to a specified level to assure a high degree of product uniformity with a range of dAT values that are readily available.
The invention will be more completely understood by reference to the following detailed description.